Global Navigation Satellite System

Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

GNSS-2 is the second generation of systems that independently provides a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation. This system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Development is also in progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS-2 system.¹

Modern systems are more direct. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.

Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.

The United States' Global Positioning System (GPS), which as of 2007 is the only fully functional, fully available global navigation satellite system. It consists of up to 32 medium Earth orbit satellites in six different orbital planes, with the exact number of satellites varying as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is currently the world's most utilized satellite navigation system.

The formerly Soviet, and now Russian, Global'naya Navigatsionnaya Sputnikovaya Sistema, or GLONASS, was a fully functional navigation constellation but since the collapse of the Soviet Union has fallen into disrepair, leading to gaps in coverage and only partial availability. India signs GLONASS agreement. India and Russia agree on joint development Of future Glonas Navigation System.

The Indian Regional Navigational Satellite System (IRNSS) is a proposed autonomous regional satellite navigation system to be constructed and controlled by the Indian government. It is intended to provide an absolute position accuracy of better than 20 meters throughout India and within a region extending approximately 1,500 to 2,000 km around it. A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India. The government approved the project in May 2006, with the intention it be implemented within six to seven years.